To the Hong Kong government, the West Kowloon station is important economically, accelerating access to mainland China and its growing high-speed rail network. But to many legal scholars and democracy supporters, it is a trap that will rob Hong Kong of a measure of autonomy, and set the stage for it to lose much more in the future.

The new rail line will cut the two-hour trip between Hong Kong and the city of Guangzhou to 48 minutes. Some critics, though, have questioned the project’s $10.8 billion cost, an increase from the original estimate of $8.3 billion, when Hong Kong already has rail links with mainland China, in addition to air, road and ferry connections.

The biggest issue, though, is not the price tag of the sweeping, clamshell-shaped station, built in western Kowloon near shopping malls, apartment towers and a new arts district. More troubling to many is that in the heart of Hong Kong, mainland Chinese law will hold sway over 26 acres of floor space in the station, an area slightly larger than Yankee Stadium.

When Hong Kong was returned to Chinese control in 1997, the arrangement with Beijing was that it would be given a “high degree of autonomy” under a model of “one country, two systems.” For 50 years Hong Kong could keep its independent courts, its free-market economy and its freewheeling press, while Beijing was responsible for matters of state like national defense and foreign relations.

Pro-democracy lawmakers on Thursday after the passage of a measure allowing mainland Chinese police officers to operate in the new rail station.CreditKin Cheung/Associated Press

The granting of mainland jurisdiction is a subtle yet powerful way to undermine Hong Kong’s unique status, said Victoria Hui, an associate professor of political science at the University of Notre Dame.

The new station “will become a horrifying precedent that it’s possible to introduce laws from China in Hong Kong without any legal basis,” said Chris Ng, a representative of the Progressive Lawyers Group, a Hong Kong organization that promotes democracy and the rule of law.

The mainland officers would have authority over a customs and immigration area that makes up about one quarter of the West Kowloon station.

Eric Cheung, a professor of law at the University of Hong Kong, said there was a risk that the passage of the station arrangement would bypass Hong Kong’s courts. But Hong Kong’s secretary for justice, Teresa Cheng, said it would have to undergo court scrutiny, though she said she believed the plan was on solid legal ground.

Pro-democracy legislators tried to limit the scope or permanence of the mainland officers with a series of amendments, but they were all defeated Thursday.

After a series of dismissals of pro-democracy legislators in the past year, the camp has lost much of its veto power. Five lawmakers were barred from the chamber after protesting the measure on Wednesday. The final vote was approved by a 40-to-20 vote as hundreds of demonstrators watched from outside.